Tuesday, April 14, 2015

The ultimate limits to carbon burning: an order of magnitude calculation

Total amount of fossil carbon on the Earth, from Vanderbroucke and Largeau (1)

During the past few years, the development of "shale gas" and "shale oil" in the US, generated a wave of optimism that spread widely in the mediasphere. It was common to hear of "a century of abundance" or even of "centuries" provided by these new sources. However, with the recent collapse of the oil market, these claims seem to have gone the same way as those of the sightings of the Loch Ness monster. But there remains a point to be made: what is exactly the limit to what we can burn? Could we really keep burning for centuries? Or, maybe, even for millennia or more?

Let's see if we can make a calculation, at least in terms of order of magnitudes. The first question is how much fossil carbon do we have on this planet. The total is reported to be about 1.5x10+16 t (metric tons), mainly in the form of kerogen, a product of the decomposition of organic matter which is a precursor to the formation of fossil fuels (gas, oil, and coal) (2) .

It looks like a lot of carbon, especially if we compare this number with the amount we are burning nowadays. The data reported by CDIAC (Carbon Dioxide Information Analysis Center) report 9.2x10+9t of carbon transformed into CO2 as the result of fossil fuel burning (gas+oil+coal) in 2013. As an order of magnitude estimate,at this rate, we could go on burning for more than a million years before truly running out of fossil carbon.

But, obviously, that's not possible. Simply, there is not enough oxygen in the atmosphere to burn all the existing fossil carbon. The total amount of free oxygen is estimated to be about 1.2x10+15tor 3.7x10+19mol O2⁠ (a "mole" is a unit used in chemistry to compare the amount of reactants in chemical reactions). One mole of molecular oxygen will react with exactly one mole of carbon to form carbon dioxide and, since 1.5x10+16tof carbon correspond 1.25x10 +21 mol, there follows that we cannot possibly burn more than about 1% of the existing fossil carbon. Instead of a million years, we are down to about 10,000 years.

Of course, then, burning that 1% of carbon would mean to use up all the oxygen of the atmosphere and that would be bad for us, no matter how much we need fossil fuels. In practice, we can't use up more than a few percent of the atmospheric oxygen; otherwise the effect on human health and on the whole ecosphere would be likely disastrous. Let's say that we are willing to bet that a 5% loss is still safe, even though nobody could be sure about that. It means that we only have 500 years or so to keep on burning before we start feeling symptoms of suffocation. But the story doesn't end here.

So far, we have been reasoning in terms of the total amount of fossil carbon as if it were all burnable, but is it? Kerogen, the main component of this carbon, can be combined with oxygen producing a certain amount of heat (3) but it can hardly be considered as a fuel, because it would be very expensive to extract and the net energy yield would be modest or even negative. In 1997, Rogner (4) carried out an extensive survey of the carbon resources potentially usable as fuel. At page 149 of this link, we can find an aggregate estimate of 9.8x10+11t of carbon as "reserves" and up to 5.5x10+12t of "resources", the latter defined as not economically exploitable at the current prices. "Additional occurrences" are reported to a possible amount of 1.5x10+13t of carbon, but that is a rather wild estimation. If we limit ourselves to proven reserves, we see that at the present rate of about 1x10+10t/year we would have about a century of carbon to go.

We are not finished, yet. We now need to consider how much carbon we can combine with oxygen before the increased greenhouse effect caused by the resulting carbon dioxide generates irreversible changes in the Earth's climate. The "tipping point" of the climate catastrophe is often estimated as that corresponding to a temperature increase of 2 deg C and, in order not to exceed it, we should not release more than about 10+12t of CO2 in the atmosphere. That corresponds to 3.7x10+11t of carbon (5). This is about one third of Rogner's global reserve estimate. So, at this point, we don't have a century any more, but only about three-four decades (and note that the estimation of what we can burn and still avoid catastrophe may have been optimistic. See also here for a more detailed estimate that takes into account different kinds of fuels).

You see how misleading it can be to list carbon resources as if they were soldiers lined up for battle. Not everything that exists inside the Earth's crust can be extracted and burned and we can't afford to extract and burn everything that could be extracted without wrecking the atmosphere. Taking into account the various factors involved, we went down from more than a million years of supply to just a few decades.

But, of course, calculating the number of remaining years at constant production rates is also misleading. In practice, fuel production rates have never been constant over history; rather, the production tends to follow a "bell shaped" curve that peaks and then declines. Today, we may be close to the peak (See e.g.here). Will the impending decline save us from catastrophic climate change? At present, we cannot say; too many are the uncertainties involved in these estimates. What we can say is that we are not facing centuries of abundance, but a decline which might even very rapid, considering the possibility of a "Seneca collapse."

In short, the age of fossil fuels is ending. It is time to take note of that and move to something else.

Hello! This is Ugo Bardi - I tend to overextend myself on the Web by writing a lot of stuff. Presently, my blog in English is titled "Cassandra's Legacy". In English, I have another blog a little more esoteric, titled Chimeras. The first is dedicated to sustainability, the second to mythology, history, and art. See also my latest book, "The Seneca Effect," Springer 2017.

Well, no.... On the average, the biosphere consumes as much oxygen as it produces. The unbalance is due mainly to the segregation of "fossil" carbon in the crust and it is a very, very slow process. Of course, if we burn this fossil, we reverse the process and we do that much faster!

And, of course, we should be thankful that carbon is being fossilized in the crust, because the Sun's output is gradually increasing as it goes through the stellar cycle, meaning that if carbon was not sequestered, we most likely would not be here. Unfortunately, not many people think on such scales...

Being a cornucopian has much to do with ignoring the second law of Thermodynamics. For them the supply curve of any fossil fuel is a flat line: no matter how much demand sways rightwards, the price is left unmoved - business as usual. In real life not all fossil carbons and are really fuels, since most are net energy negative; others may be slightly positive but are uncompetitive with alternative energy sources.

I am not very impressed. The kind of models they use are oversimplified and may easily diverge from reality. They found that the oxygen concentration may go to zero in relatively short times, and that has never happened during the whole Phanerozoic. So, I wouldn't worry too much, unless we really manage to do something horrible to the ecosystem..... which in the end we might!

Thanks Ugo,I appreciate that reality check. You confirmed what I suspected. Other than looking past the idiots that post garbage from reddit, like the billybub or whatever handle that person who referred to that is, I have made a projection that includes feedbacks as well. I believe that with a Paris agreement, temperatures by 2100 will hold at 2.5-3.5C, give or take. Do you think I'm in the right neighborhood, and what is your opinion on possibly using SRM to keep temperatures stable if the need arises?

Ugo Bardi's blog

This blog deals with the future of humankind in view of such things as the overexploitation of natural resourecs and the effects of global warming. It is a bit catastrophistic, I know, but, after all, the ancient prophetess, Cassandra (above in a painting by Evelyn de Morgan) turned out to have been right!

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Listen! for no more the presage of my soul, Bride-like, shall peer from its secluding veil; But as the morning wind blows clear the east,More bright shall blow the wind of prophecy,And I will speak, but in dark speech no more.(Aeschylus, Agamemnon)

The Seneca Effect

The Seneca Effect: is this what our future looks like?

Chimeras: another blog by UB

Another blog by Ugo Bardi; it is dedicated to art, myths, literature, and history with a special attention to ancient monsters and deities.

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I try to publish at least a post every week, typically on Mondays, but additional posts often appear on different days. Comments are moderated: no insults, no hate, no trolls. You may reproduce my posts as you like, citing the source is appreciated!

About the author

Ugo Bardi teaches physical chemistry at the University of Florence, in Italy. He is interested in resource depletion, system dynamics modeling, climate science and renewable energy. Contact: ugo.bardi(whirlything)unifi.it